Add Coveralls integration

[blag]

Add Coveralls integration so we can track coverage over time, as well as enable contributors to see how their changes affect code coverage.

[PetrDlouhy]

It is interesting, that Coverals report the Django <5.0 branch of _get_flatchoices untested:

I am not sure why that happened.

[PetrDlouhy]

@blag I think that there must be some kind of problem with the different Django versions. While in Django 5.0 the tree looks like this:

On Django 4.2 the tree looks like this: And the results are not accessible.

Maybe it uses different configuration with each Django version.

Also on Django 5.0 are examples part of the results, but I think they should not be.

[goinnn]

Added coveralls integration with last changes.

@PetrDlouhy and @blag you need combine all coverages: https://stackoverflow.com/questions/19274492/how-calculate-the-global-coverage.

Now this line is tested:

        flat_choices = super()._get_flatchoices()